As DSFR now functions with a VSS snapshot, you will be limited by how fast you read the snapshot(s) which in turn depends on how much activity there is on the original volume(s). One thing to be aware of is that VSS is pretty heavy on the virtual memory, so a bigger pagefil can help.
Playing with the buffer and raw buffer size might give some improvement.
There also might be a setting in windows which tells it not use VSS for backup/restore of DSFR
And you can consider to stop the DSFR service before the backup and starting after, like you did in the earlier days.